The present invention relates in general to semiconductor devices and the method of fabrication thereof. More particularly, the present invention relates to an improved technique for the fabrication of rugged semiconductor beam lead devices while maintaining proper microwave performance. Even more particularly, the invention relates to an improved method of fabrication of Schottky beam lead diodes featuring a solid dielectric frame preferably of polysilicon encompassing the entire active silicon region and providing exceptional physical durability at no sacrifice to microwave performance.
One problem with conventional beam lead devices is that they are extremely fragile. They can be easily destroyed by a force of about 3 grams which is roughly equivalent to the weight of a dime. This means that even if wafer yields are good, production yields suffer because devices are destroyed in transit, during testing, and when assembled into the systems for which they are intended. Because these devices may be used in large quantities it is highly desirable to have a high assembly yield. This is particularly necessary when semi-skilled people are being used for assembly which many times is the case.
The conventional beam lead device employs a glass for support, typically frit glass. By way of example, see British Patent 2,028,583 that describes the use of a glass frit for beam lead support. However, glass is a notoriously difficult material to work with. Also, parasitic capacitances can vary significantly from device to device often forcing the use of sorting procedures before shipment to a customer. One of the disadvantages of the use of a frit glass is that the glass must be fired at fairly high temperature. When this is done, the final electrical performance of the device suffers due to outdiffusion from the heavily doped substrate into the lightly doped epitaxy.
Another problem with frit glass is that it is difficult to machine. It must be machined after it has been applied to the wafer to replanarize the wafer. If this is not done, high resolution photolithography becomes impossible to perform because, without lapping, the wafter has a non-planar surface. However, when the surface is machined, other difficulties arise: it is very easy to machine part of the epitaxy away.
Moreover, the glassed beam lead devices are expensive because of the costly processing, which requires many complicated steps when working with frit glass. For example, severe problems occur when a design calls for fritted glass and requires the glass to be mixed in solution, to be applied and fired and to be defined within a very strict, fine geometry. It is difficult to carry out and yields problems primarily regarding the parasitic capacitance of the device. If a number of wafers are examined, for example, some will have adequate capacitance while others will have capacitance that is too high or too low.
Other problems associated with the use of glass include the fact that it is not a semiconductor material and is thus not compatible with the semiconductor substrate. Frit glass has substantial impurities and when used in the usual high temperature processing causes wafer contamination. The glass frit also has poor thermal match to a silicon based wafer.
Accordingly, it is an object of the present invention to provide an improved semiconductor device and associated method of fabrication for in particular providing a beam lead device that is exceptionally rugged in construction.
Another object of the present invention is to provide an improved method of fabrication of a beam lead type device without requiring the use of a glass support but which instead employs a polysilicon deposition technique which has been found to be substantially more process-friendly than with the use of a frit glass.
A further object of the present invention is to provide an improved beam lead semiconductor device and associated method of fabrication in which the device has high wafer yields and production yields.
Still another object of the present invention is to provide a rugged beam lead device, preferably a Schottky beam lead device that is far less likely to be damaged than with existing devices. In accordance with the present invention the beam strength of the diodes exceed 10 grams and are typically 20 grams while with the use of a conventional glass frit the beam lead strength is 4 grams or less. Many standard planar beam lead diodes are generally rated at 4 grams or less.
A further objection of the present invention is to provide an improved semiconductor device and method of fabrication in which the method can be carried out for fabricating many different types of semiconductor devices such as in a balanced-mixer application where two of the beam lead devices are utilized. The principles may also be used in microwave detectors or high speed switches.
A further object of the present invention is to provide an improved technique for the fabrication of beam lead devices particularly Schottky beam lead diodes and in which the manufacturing can be carried out at a lower cost than presently possible.
Still another object of the present invention is to provide an improved semiconductor device and associated method of fabrication in which the device has almost perfect reproducibility from device to device and associated uniformity. This is carried out in accordance with the process of the present invention by controlling process steps so that the same device configuration is readily reproducible from device to device.
A further object of the present invention is to provide an improved beam lead device and associated process where each of the photo steps and the other steps in creating the device are carried out in a manner essentially operator-independent. This produces a process that results in high yield, and remarkable uniformity in each wafer and between wafers.